This investigation is concerned with changes in the transmission properties of invertebrate sensory interneurones, both their physiological causation and their significance to behaviour. Both changes in responsiveness consequent on performance (e.g. habituation and dishabituation) and also "spontaneous" fluctuations in responsiveness are studied. The behavioural significance is assessed by recording from chronically implanted, freely moving animals, and also by the elucidation of the output connections of the labile units. The synaptic mechanisms of the observed changes are investigated by conventional intracellular and extracellular techniques. An especial aim is to assess the extent to which these effects are ascribable to efferent innervation, and the possible existence of integrating mechanisms corresponding to the general arousal systems known in the vertebrate CNS. Most of this work is performed on insects, but gastropod and cephalopod molluscs are also used.